The tuning input of a voltage controlled oscillator (VCO) has been used to generate a modulated transmit signal. However, when the VCO is included in a phase locked loop (PLL) configuration, the response of the PLL acts as a high pass filter, effectively removing low frequency components of the modulation signal. To eliminate this effect, singlepoint modulation schemes were developed that inject the modulating signal by either adjusting the VCO input or by adjusting a frequency divider ratio in the feedback loop. One limitation of such systems is that the bandwidth of the modulation is inherently limited by the bandwidth of the PLL feedback loop. Even with the addition of high-pass compensation schemes, the bandwidth available is limited to a few megahertz. Two-point modulation schemes were developed that inject the modulating signal at the controllers of both the VCO and the frequency divider in the feedback loop such that the high-pass characteristics at the VCO input is compensated by the low-pass characteristics at the divider control.
FIG. 1 is a prior art system 100 for providing a phase-modulated signal 102 processed by a differentiator 104 that is then applied to a digitally controlled oscillator (DCO) 106. The output of the DCO is applied to a loop filter 108 and combined at the input to the DCO 106. The output 110 is applied to an amplifier 112 to generate an amplified transmit signal 114.
FIG. 2A is a signal plot showing the frequency of an original phase modulated signal and the frequency of the signal after high-pass filtering. Although the frequency signals 202, 204 appear very close, the plots of FIG. 2B show that the phase 210 of the output signal differs significantly from the input phase 206. Initially, in the example waveform of FIG. 2B, the input phase 206 is greater than the output phase 210 until the waveforms cross approximately at point 208.
To compensate for the limitations of the single-point modulation, so-called “two-point” modulation schemes were developed to increase the available bandwidth that may be generated by the PLL/VCO modulator. As shown in FIG. 4, two-point systems typically operate by splitting the information signal into high-pass 402 and low-pass 404 components, where the high-pass component is used to direct-modulate the VCO, while the low pass component is used within the PLL loop to either adjust the phase detector output or to adjust a frequency divider ratio in the feedback loop. Modulation at these two points produces a composite signal 406 with a spectral response (as shown by the Power Spectral Density, or PSD) extending down to DC as shown in 408.
Some inherent challenges exist with two-point modulation, including synchronization of the high-pass and low-pass modulation components, matching the gains on the high-pass and low-pass modulation paths, as well as high power consumption resulting from high-rate PLL operations.
Accordingly, there is a need for improved wideband two-point modulators.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.